Chartjs-Where is the documention for the chart.js Chart constructor?

The Chart.js documentation, that you linked to, is the comprehensive documentation you’re looking for. The only more comprehensive (and authoritative) source of documentation is the source code itself.

As you wrote, the second parameter is a complex object. It could, potentially, be dozens (or hundreds!) of lines in length. Also consider that not every option is applicable to every chart so showing them together would be misleading.

Trying to document such complexity without breaking it down into separate sections would be a fools errand.